1. Field of the invention
This invention relates to aramid paper which is used as electrical insulating sheet (paper) for transformers, electric motors, generators, etc. and which is excellent in machine characteristics, resistance to heat, electrical insulation properties and chemical resistance, a method of manufacturing the aramid paper, and an aramid-polyester laminate.
2. Description of the related art
Heat-resistant synthetic insulating paper called “aramid paper” has recently been provided as an electrical insulating material. More specifically, for example, aromatic polyamide paper is known under the trademark of Nomex® owned by E. I. du Pont de Nemours and Company, U.S.A. (hereinafter, DuPont), for example, Nomex® #410 and #411. This aromatic polyamide paper has excellent machine characteristics and electrical insulation properties and has a thickness ranging from 2 to 20 mil. This aramid paper is used as an insulating material for transformers, electric motors and generators all of which necessitate high heat resistance performance belonging to type H (180° C.) in a heat resistance division of International Electrotechnical Commission (IEC) 85 (1984).
This aramid paper is made from masticated aramid (aromatic polyamide) fibrid and heat-resistant short fiber as main constituents by a wet papermaking process which is similar to a method of making Japanese paper. Furthermore, if necessary, these main constituents are heated and pressurized (calendaring) thereby to be commercialized. The aforenoted Nomex® #410 is a calendered product, whereas the aforenoted Nomex® #411 is a non-calendered product. Each of these aramid papers has a number of voids therein as normal paper does. Accordingly, each aramid paper necessarily has a lower electrical breakdown strength (KV/mm) per thickness than a film having the same quality and same thickness.
On the other hand, a polyester film (hereinafter, “PET film”) such as polyethylene terephthalate or polyethylene naphthalate has a lower resistance to heat than the aramid paper and is classified into a type E (120° C.) in the heat resistance division of IEC 85 (1984). However, the PET film has a higher electrical breakdown strength and cost-effectiveness. The PET film is used for the purpose of insulation corresponding to the type E or lower types in a vast range.
Furthermore, insulating materials belonging to the type F (155° C.) have recently been desired. The insulating materials of the type F require lower resistance to heat than those of type H. Aramid paper has no problem if used in type F but is expensive. As a result, development of cheaper materials has been desired. The following materials have conventionally been proposed as cost-effective F type materials:
(a) A multilayer structure of aramid paper having high resistance to heat and to oxidation and a PET film having high electrical insulation. The aramid paper and the PET film are bonded together by an adhesive agent;
(b) An aramid laminate made by laying aramid paper and a PET film on each other and pressurizing and heating the laid aramid paper and PET film under the conditions of temperature ranging from 220° C. to 250° C. and linear pressure of 50 kg/cm or above so that the laid aramid paper and PET film are laminated by thermal adhesion thereby to be laminated. See JP-H07-32549A (1995), for example. The used aramid paper is meta-aramid paper, whereas the used PET film is a biaxially oriented PET film; and
(c) A laminate made by laying on the surface of an aramid paper layer (layer A) a layer of PET welded or impregnated at the melting point and another PET film. Thereafter, both layers laid on each other are welded together at a roll temperature of 220° C. to 250° C. and pressure of 50 kg/cm or above and then quenched at 100° C./minute or above, thereby obtaining the laminate. See JP-H07-299891A (1995), for example.
In the above case (a), the aramid paper and PET film are affixed on each other by the adhesive agent. Since the adhesive agent is relatively harder, the elasticity of each of the aramid paper and PET film is damaged by the harder adhesive agent, whereupon the multilayer structure is disadvantageous in workability such as bending. Furthermore, when the multilayer structure is applied to oil-immersed equipment, components of the adhesive agent may be transferred to the oil, whereupon the usage of the multilayer structure is limited.
On the other hand, the aramid paper and the PET film are bonded together by thermal welding without use of an adhesive agent, so that the defect resulting from use of adhesive agent can be overcome. In the case (b), however, since the temperature of thermal welding is approximate to a melting point of PET (about 260° C.), variations in the dimensions of PET film are increased. This results in warpage, shrinkage or crease in the welded products. Furthermore, part of PET tends to be crystallized. As a result, it is difficult to obtain a stable quality. Since the temperature of thermal welding is also high in the above case (c), too, part of PET impregnated in the aramid paper is crystallized such that the excellent elasticity is damaged.